Telephone system



c. E. LoMAx TELEPHONE SYSTEM Filed 00L-31. 1942 l w i www? $2 im M mm/ mm J omum Om mOFOmjmw May 9, 1944.

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B Q .228 20E Patented May 9, 1944 TELEPHONE SYSTEM Clarence E. Lomax, Chicago, lll., assignor to Automatic Electric Laboratories, Inc., a corporation oi' Delaware Application October 31, 1942, Serial No. 463,998

16 Claims.

' independent units, one unit terminating the line conductors of the various lines and the other unit terminating the `control conductors of the various lines, the two units being arranged one above the other inA spaced-apart relation. Also the dropback selector comprises a wiper set including line land control wipers arranged in spaced-apart relation one above the other and adapted to cooperate with the two respective units of the contact bank. Further the wiper set has a normal position and is mounted for both primary or vertical and secondary or rotary step-by-step movements with respect to the associated contact bank.

In a small automatic telephone system of the type noted, it has been the customary practice to employ standard Strowger drop-back selectors in spite of the fact that only a fraction of the total number of individual contacts in the two -units of the contact. bank terminate the-respective line and control conductors of the various lines; whereby there is a considerable number of unused contacts in the contact bank.

Accordingly, it is an object of the present invention to provide in a small automatic telephone system an improved drop-back seelctor which effects a considerable saving in the total number of contacts provided in the contactl bank thereof, whereby only a minimum number of idle contacts appear therein, and which is of compact and economical construction and arrangement.

Another object of the invention is to.l provide a drop-back selector of thc type noted, which comprises a contact bank including onlyv one unit which terminates both the lline and control conductors of the various lines.

A further object of the invention isjto provide a dropiback selector of the type noted which also comprises a wiper set including line jand control wipers which are adapted to cooperate with two respective sections of t'he associated unitary contact bank, whereby both the line and control wipers are moved adjacent the two respective sectionsof the contact bank only in the event a predetermined amount of primary movement is imparted to the wiper set, and wherein the wiper set is returned back to its normal position atthe conclusion ofy the primary movement thereof in the event less than the predetermined amount of primary movement is imparted thereto.

` 'Further features of the invention pertain to the particular arrangement of the circuit elements of the system, whereby the above-outlined and additional operating features are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best beunderstood by reference to the `following speciiication taken in connection with the accompanying draw ing, in which the single flgure illustrates the arrangement of a telephone system incorporating a drop-back selector embodying the present invention.

Referring now more particularly to the single figure of the drawing, there is illustrated a small automatic telephone system comprising an exchange terminating `asvmany as four hundred subscriber lines, including the subscriber lines I0 and I2 illustrated. Also the system comprises an operator position I3 terminating as many as ten trunks, including the trunk I4. The subscriber lines III and I2 extend to subscriber substations TI and T2, respectively, and terminate in individually associated line switches, the subscriber line Ill terminating in the individually associated line switch II and the subscriber line I2 terminating in the individually associated line switch, not shown. Each of the trunks I4, etc., terminating at the operator position I3 alsoterminates in a trunk circuit individual thereto, the

trunk I4 terminating in the individually associated trunk circuit l5. Further, each of the trunk circuits I5, etc., is provided with an incoming trunk individual thereto, the trunk circuit I5 being provided with the incoming trunk 20 individual thereto.

Further the exchange comprises a group of drop-back selectors, including the drop-back selector 30, four groups of connectors, the rst group of connectors including the connector 24.

Each of the line switches Il, etc., h as access to each of the drop-back selectors 30,\ etc.; and each of the drop-back selectors 30, etc., has access to each of the incoming trunks 20, etc., and to each of the groups of connectors. For example, the drop-back selector 30 has access to the incoming trunk 20 extending to thetrunk circuit I5 and to a trunk 25 extending to the connector 24 included in the nrst group. Finally, each connector in each group has access to one hundredv of the associated subscriber lines. For example, the

Preferably, each of the drop-back selectors is identical to the drop-back selector 34 illustrated, which comprises a Strowger mechanism 40 including a single unitary contact bank 45; the contact bank 45 including ten longitudinally or vertically spaced-apart laterally or circumferent.ally extending rows of ten pairs of lcontacts each, wherein each two adjacent rows of contacts are spaced apart a longitudinal or vertical distance substantially V and each two adjacent individual pairs of contacts in each row are spaced apart a lateral or circumferential distance substantially R. In the contact bank 45, the ten individual pairs of contactsin the first level or row terminate the line conductors of the ten trunks in the first group extending to the first group oi' connectors, including theconnector 24; while the lower contacts of the ten individual pairs of contacts inthe corresponding sixth level or row terminate the control conductors of the ten trunks in the rst group extending to the first group of connectors, including the connector For example, the tenth pair of contacts in the first level or row terminates the line conductors C25 and C21 of the trunk 25 extending to the connector 24; while the lower contact of the tenth pair of contacts in the corresponding sixth level or row terminates the control conductor C28 of the trunk 25 extending to the connector 24. Similarly, in the contact bank 45, the ten individual pairs of contacts in the second, third and fourth levels or rows terminate the line conductors of .the ten trunks in the respective second, third and fourth groups extending to the respective second, third and fourth groups of connectors: while the lower contacts of the ten individual pairs of contacts in the corresponding seventh, eighth and ninth levels or rows terminate the control conductors of the ten trunks in the respective second, third and fourth groups extending to the respective second, thirdV and fourth groups of connectors. Finally, in the contact bank 45, the ten individual pairs of contacts in the fifth level or row terminate the line conductors `of the ten trunks in the group extending to the group of trlmk circuits, including the trunk circuit I5; while the lower contacts of the ten individual pairs of contacts in the corresponding Ii level or row terminate the control conductors of the ten trunks in the group extending to the group of trunk circuits,

A including the trunk circuit I5. For example,

the tenth pair of contacts in the fifth level or row terminates the line conductors C2i and C22 of the trunk 20 extending to the trunk circuit I5; while the lower contact of the tenth pair of contacts in the corresponding'. level or row terminates the control conductors C22 of the trunk 20 extending to the trunk circuit I5. A

Further the Strcwger mechanism 44 comprises a wiper shaft 4| having a normal position and mounted for both ionsitudinai or vertical and lateral or rotary step-by-step movements. vThe wiper shaft 4| carries a wiper set including an upper pair of control wipers 44 and 44' and a lower pair of line wipers 42 and 42; the upper pair of control wipers being spaced apartwith respect to the lower pair of line wipers a longitudinal or vertical distance substantially KV, wherein K is the whole number 5. Accordingly, the upper pair of control wipers is spaced from the lower pair of line wipers a distance substantially 5V; whereby the upper and lower pairs of wipers are adapted to cooperate simultaneously with the levels 5 and I, 1 and 2, 5 and l, I and 4, 0 and 5. Also it is noted that, when the Wiper shaft 4| occupies its' normal vertical and rotary positions, the control wiper 44 is spaced longitudinally or below the first level in the contact bank 45 the distance substantially V and is spaced laterally or circumferentially from the first pair of line contacts in the first level in the contact bank 45 the distance substantially R.

At this point it is noted that the upper control wiper 44' carried by the wiper shaft 4| is not utilized in the present arrangement. Similarly, the upper contacts of the various pairs in the sixth, seventh. eighth, ninth and l levelsin the 'contact bank 45 are not utilized in the present arrangement. Finally the contact bank 45 comprises a lower line contact section including the first five levels I to 5, inclusive, and an upper control contact section including the second five levels 6 to l, inclusive.

Further, the Strowgermechanism 40 comprises a vertical magnet M45 operative to drive the wiper shaft 4I step by step in the longitudinal or vertical direction the distance substantially V each step; a rotary magnet M41 operative to drive the wiper shaft 4| step by step in the lateral or rotary direction the distance substantially R each step: and a release magnet M4! operative to release the wiper shaft 4| and to cause it to be returned to its normal vertical and rotary positions. Also associated with, the Strowger mechanism 40 are three sets of switch springs S54, S5| and S52 which are actuated when the wiper shaft 4I is driven in the vertical direction away from its normal vertical position: two sets of switch springs S52 and 854 which are actuated when the wiper shaft 4| is driven in the rotary direction eleven steps away from its normal rotary position: and a set of switch springs S51 which is actuated when the wiper shaft 4| is driven in the vertical direction six, seven, eight, nine or ten steps away from its normal vertical position. i

Further the drop-back selector 20 comprises a relay group. including a switching relay R50, a line relay R10, a hold relay R and a transfer relay R; a busy tone transformer 92; a dial tone transformer 93: and a control network connected and arranged in a manner more fully described hereinafter.

In View of the above description of the line terminal arrangement in the contact bank 45 of the Strowger mechanism 40, it will be understood that the directory number of the 'operator position I3 comprises the single digit 0; the directory number of the first, second, third and fourth groups o f one hundred subscriber substations respectively comprise the digits 53X, 1XX, IXX and SXX; while the set of switch springs S51 is not actuated when the first digit received by the dropback selectorl 20 is 1, 2, 3, 4 or 5, whereby the wiper shaft 4l of the Btrowger mechanism 40 is operated a, corresponding number of steps in the vertical direction away from its normal vertical position and is then dropped back to its normal vertical position. Accordingly, the digits 1, 2, 3. 4 and 5 are not assigned as. rst digits in the telephone system.

Considering now the general operation of the telephone system illustrated, a call may be initiated at any one of the subscriber substations, such, for example, as the subscriber substation Tl. When the call is initiated at the calling subscriber substation TI, when the subscriber thereat removes the receiver of the telephone instrument `from its associated switchhook, a circuit is closed between the calling subscriber substation Tl and the line switch H via the subscriber line i0, whereby operation of the line switch II is initiated. The line switch Il operates to seize an idle one of the drop-back selectors in the associated group, such, for example, as the drop-back selector 30. More particularly, the line switch I l seizes the drop-back selector 30 due to the absence of ground potential and the presence of battery potential upon the control terminal thereof; battery potential being applied by way of the lower winding of the transfer relay R90 and the set of switch springs S50 to the control terminal of the drop-back selector 30. When the line switch H seizes the line and control terminals of the drop-back selector 30, ground potential therein is applied to the control terminal, thereby to complete an obvious circuit, including the set of switch springs S50, for energizing the lower winding of the transfer relay R90, whereby the line switch Il is switched through. When thus energized the transfer relay R90 operates. Also when the line switch Il seizes the line terminals of the drop-back selector 30 a loop circuit is completed for energizing in series the upper and lower windings of the line relay R10; the above-mentioned circuit extends from ground by way of the secondary winding of the dial tone transformer 93, the contacts 56 of the set of switch springs S54, the lower winding of R10 and the contacts 63 to the positive line terminal, and from battery by way of the upper winding of R10 and the contacts 6I to the negative line terminal, the positive and negative line terminals being connected by way of the line switch Il and the subscriber line l to the calling subscriber substation Ti. When thus energized the line relay R operates and dial tone` current is returned over the above-traced loop circuit from the secondary winding of the dial tone transformer 93, whereby the receiver of the telephone instrument at the calling subscriber substation TI is operated in accordance with the dial tone in order to indicate'to the calling subscriber thereat that he may proceed with the dialing of the directory number. Upon operating, the line relay R10 completes, at the contacts 1|, an obvious circuit, including the contacts 91, for energizing the winding of the hold relay R80,

thereby to cause the latter relay to operate and y complete, at the contacts 9|, an obvious multiple holding circuit for energizing the lower winding of the transfer relay R90 and anv obvious path for applying ground potential to the control terminal. The application of ground potential to the control terminal retains the line switch Il in its operated position. Further the hold relay R80 completes, at the contacts 8l and 82,

an obvious path for applying ground potential by way of 'the upper Winding of the switching relay R50 to the control wiper 44 of ,the wiper set for test purposes, in a manner more fully explained hereinafter. Finally the hold relay R completes, at the contacts 83, an obvious circuit for energizing the intermediate winding of the switching relay R60, thereby to render the latter relay exceedingly fast to operate when the upper winding thereof is subsequently energized, the switching relay R60 being of the polarized type. At this time the drop-back selector 30 has been seized by the line switch Il and is in readiness to receive the first digit of the directory number dialed at the calling subscriber substation`Tl.

Now assume that the subscriber at the calling subscriber substation TI dials the first digit of a directory number. When the subscriber at the calling subscriber substation TI operates the associated dial in accordance with any digit, a corresponding number of impulses are transmitted over the loop circuit to the line relay R10, whereby the latter relay follows the impulses of the' digit mentioned. Each time the line relay R10 restores and-then recperates it interrupts and then recompletes, at' the contacts 1|, the previously mentioned circuit for energizing the winding of the hold relay R80, whereby the latter relay remains operated during impulsing as it is of the slow-to-release type. Also each time the line relay R10 restores and then reoperates it completes and then interrupts, at the contacts 12, a' multiple circuit, including the contacts 61 and 84,' for energizing the upper winding of the transfer relay R and the vertical magnet M46. whereby thevertical magnet M46 operates and restores a corresponding number of times in order to drive the wiper shaft 4I a corresponding number of steps in the vertical direction away from its normal vertical position; and whereby the transfer relay R90 is retained operated during impulsing as it is of the slow-to-release type. When the wiper shaft 4I is driven one step in the vertical direction the sets of switch springs S50, SEI and S52 are actuated. More particularly, the set of switch springs S50 is actuated into disengagement, thereby to interruptthe previously traced original circuit for energizing the lower winding of the transfer relay R90; the set of switch springs S5| is actuated into engagement, thereby to prepare a common point in circuits traced hereinafter for respectively energizing the rotary magnet M41 and the release magnet M49; and the set of switch springs S52 is actuated into engagement, thereby to prepare a point in an alternative circuit traced hereinafter for energizing the release magnet M49.

At the conclusion of the first digit dialed at l the calling subscriber substation TI, the line relay R10 and the hold relay R80 are retained in their operated positions and shortly thereafter the transfer relay R90 restores, the latter relay being of the slow-to-release type as previously noted. Upon restoring, the transfer relay R90 completes, at the contacts 9|, one of the previously mentioned circuits for energizing either the rotary magnet M41 or the release magnet M49, depending upon whether or not the set of switch springs S51 was actuated in response to the movement of the wiper shaft 4| in the vertical direction away from its normal vertical-position in response to the reception of the first digit by the first digit of the directory number received by the drop-fback selector 30 is one of the digits f Now assuming that the first digit Qi the directory number dialed at the calling subscriber substation Ti was one of the unassigned digits 1 to 5, inclusive, such, for example, as the digit 5, the drop-back selector 30 operates in response to the first digit 5 in the manner explained above.

, whereby the. wiper shaft 4| is driven flve steps in the vertical direction away from its normal vertical position and the set of switch springs S51 is atea the dial in accordance with the proper and assigned first digit of the directory number.

Now assuming that the first digit of the directory number dialed at the calling subscriber substation TI was one of the assigned digits 6 to 9, inclusive, such, for example, as the digit 6, the drop-back selector 39 operates in response to the first digit 6, in the manner explained above,

. whereby the wiper shaft 4| is driven six steps not actuated. At this time the pair of ycontrol 1 wipers 44 and 44' of the wiperset is disposed adjacent the flftlrlevel in the associated contact bank 45 and the pair of line wipers 42 and 43 of the wiper set is disposed the vertical distance substantially V below the first level in the associated contact bank 45. Accordingly. the wiper set carried by the wiper shaft 4| does not occupy a position wherein it can select a trunk and the Strowger mechanism 40 is released, whereupon it is dropped back to its normal vertical position incident to the restoration of the transfer relay R90. More specifically, in the present example, shortly following the conclusion of the first digit 5 dialed at the calling subscriber substation TI, the transfer relay R90 restores to complete, at the contacts 9|, the previously mentioned circuit for energizing the release magnet M49. The

.above-mentioned circuit extends from ground by way of the contacts 61 and 1|, the set of switch springs 85|, the contacts 9| and 40, the contacts V58 of the set of switch springs S51 and the release magnet M49 to battery. When thus energid the release magnet M49 operates to release the wiper shaft 4|, whereby it is returned to its normal vertical position. When the wiper shaft 4| is thus released the sets of switch springs S50, 85| and S52 are again actuated. More particularly, the set of switch springs S50 is actuated into engagement, thereby to recomplete the previously traced circuit for energizing the lower winding of the transfer relay R90, whereby the latter relay reoperates: the set of switch springs S5| is actuated into disengagement, thereby to interrupt the above-traced circuit for energizing the releasamagnet M49, whereby the latter magnet restores; and the `set of switch springs S52 is actuated into disengagement, thereby to interrupt a further point in the previously mentioned alternative circuit for energizing the release magnet M49. IAt thistime lthe above-traced loop circuit between the calling subscriber substation TI and the line relay R10 including the secondary winding of the dial-tone transformer 90 4is still completed, whereby dial tone current is still returned to the callingsubscriber substation TI in order to indicate to the subscriber thereat that the first digit of the directory number dialed was an unassigned digit. Also at this time the Strowger mechanism40 is completely released and the apparatus incorporated in the drop-back selector occupies its initial seized position, whereby the drop-back selector I0 is again prepared to receive the first digit of a directory number dialed at the calling subscriber `substation TI. Accordingly, in this case, the subscriber at the calling subscriber substation in the vertical direction away from its normal vertical position and the set of switch springs S51 is actuated. At this time the pair of control wipers 44 and 44' of the wiper set is disposed adjacent the sixth level in the associated contact bank 45 and the pair of line wipers 42 and 43 of the wiper set is disposed adjacent the first level in the associated contact bank 45. Accordingly, the wiper set carried by the wiper shaft 4| occupies a position wherein it can select a trunk and the Strowger mechanism 40 is not released incident to the restoration of the transfer relay R90. More particularly, the wiper set carried by the wiper shaft 4| occupies a position select- ,K ing the group of trunks extending to the first TI again oper- 7o group of connectors, including the connector 24: and operation of the rotary magnet M41 is initiated incident to the restoration of the transfer relay R90. More specifically, in the present example, shortly following the conclusion of the first digit 6 dialed at the calling subscriber substation TI, the transfer relay R restores to complete, at the contacts 9|, the previously mentioned circuit for energizing the rotary magnet M41. The above-mentioned circuit extends from ground by way cf the contacts 61 and 1|, the set of switch springs S5I, the contacts 9| and 48, the contacts 59 of the set of switch springs S51 and the rotary magnet M41 to battery. When thus energized the rotary magnet M41 operates to drive theWiper shaft 4| one step in the rotary direction, whereby the idle or busy condition of the first trunk in the previously selected group is tested. More particularly, in the event the trunk mentioned is respectively idle or busy, batteryor ground potential respectively appears upon the control conductor thereof and is applied to the control wiper 44 of th'e wiper set.

Assuming that the first trunk in the previously selected group is busy, ground potential appears upon the control conductor thereof and is applied to the control wiper 44 of the wiper set, as noted above; thereby to complete an obvious path, including the contacts 82 and 8|, for shortcircuiting the upper winding of the switching relay R90 in order positively to prevent operation of the latter relay at this time. Also, upon operating, the rotary magnetM41 interrupts, at the contacts 40, the previously traced circuit for energizing the rotary magnet M41, whereby the latter magnet restores. Upon restoring, the rotary magnet M41 again completes, at the contacts 40, the previously traced circuit for energizing the rotary magnet M41, whereby it reoperates in order to drive the wiper shaft 4| an additional step in the rotary direction whereby the second trunk in the previously selected group is tested. In view of the foregoing explanation.'

connector 24 in the rst group is the nrst idle trunk in the previously selected group, intermittent operation of the rotary magnet M41 continues until the wiper set selects the tenth contacts in the sixth and iirst levels respectively terminating the control conductor C28 and the line conductors C26 and C21 thereof. At this time battery potential appears upon the control conductor C28 of the trunk 26 extending to the connector 24 and terminated by the lower contact in the tenth pair in the sixth level of the contact bank 45 engaged by the control wiper 44 of the wiper set. Accordingly, battery potential is applied to the control wiper 44 of the wiper set. thereby to complete a circuit substantially identical to that previously traced and including the contacts 82 and 8l for energizing the upper winding of the switching relay R60. When thus energized the switching relay R60 operates exceedingly fast to complete, at the contacts 66, an obvious holding circuit, including the contacts 8l, for energizing the lower` winding thereof. Also the switching relay R60 completes. at the contacts 06, an obvious path, including the contacts 82, for short-circuiting the upper winding thereof and an obvious multiple path, including the contacts 8|, for applying direct ground potential to the control wiper 44 of the wiper set. Further the switching relay R60 interrupts, at the contacts 61, the previously traced circuit for energizing the rotary magnet M41, thereby positively to arrest further operation thereof and consequent movement of the wiper shaft 4I in the rotary direction. Also the switching relay R60 interrupts, at the contacts 6I and 63, the previously traced loop circuit for energizing the upper and lower windings of the line relay R10, thereby to cause the latter relay to restore. Flnally the switching relay R60 completesat the contacts 62 and 64, an alternative loop circuit the intermediate winding of the switching relay R80; however, the latter relay remains in its operated position at this time dueto the com` pleted holding circuit for energizing the lower winding thereof. Also the hold relay R80 interrupts, at the contacts 8|, the previously traced original path for applying ground potential to the control terminal of the drop-back selector and prepares, at the contacts 86, a further point in the previously mentioned alternative circuit for energizing the release magnet M48. At this time the previously traced loop circuit is completed between the calling subscriber substation TI and the connector 24; and the connector 24 is in readiness to receive' the second digit of the directory number.

The subscriber at the calling subscriber substation TI then dials the second and third digits of the directory number over the previously traced loop circuit, whereby the connector 24 is operated in accordance with conventional pracextending between the calling subscriber substation TI and the connector 24. The abovementioned loop circuit extends from the positive line conductor C26 of the trunk 25 by way of the line wiper 42 and the contacts 64 to the positive terminal of the drop-back selector 30 and from the negative line conductor C21 of the trunk 25 by way of the line wiper 43 and the contacts 62 to the negative terminal of the drop-back selector 30; the positive and negative terminals of the drop-back selector 30 being connected by way of the line switch II and the subscriber line I0 to the callingfsubscriber substation TI and ground and battery potential being applied in the connector 24 to the positive and negative line conductors C26 and C21, respectively, of the trunk 25. When the above-traced loop circuit is completed vbetween the calling subscriber substation ',II andthe connector 24. the connector 24 operates to cause .ground potential to be applied to the control conductor C28 of the trunk 25 and is conditioned to be responsive to the second digit of the directory number dialed over the above-traced loopv circuit at the calling subscriber substation TI. The application of ground potential in the connector 24 to the control conductor C28 of the trunk 25 completes an alternative path. including the control wiper 44 of the wiper set and the contacts 65 of the operated switching relay R60, for applying ground potential to the control terminal of the drop-back selector 30, whereby the line switch I I is retained in its operated position.

Upon restoring, the line relay R10 interrupts. at the contacts 1 I the previously mentioned cirtice to select the corresponding subscriber line, such, for example, as the subscriber line I2, extending to the called subscriber substation T2. The connector 24 then operates in accordance with conventional practice to test the idle or busy condition of the selected subscriber line I2,

whereby busy tone current is returned over the previously traced loop circuit to thecalling subscriber substation TI in the event the called subscriber substation T2 `is busy at this time. On the other hand,v the connector 24 operates in order to project ringing current over the subscriber line I2 to the called subscriber, substation T2 and ring-back tone current over the previously traced loop circuit to the calling subscriber substation TI in the event the called subscriber substation T2 is idle at this time. Assuming that the called subscriber substation T2 is idle at this time, when the subscriber thereat answers the call by removing the receiver of the telephone instrument from its associated switchhook, an obvious connection is completed between the calling subscriber substation TI and the called A subscriber substation T2; this connection including the subscriber line I0, the line switch II. the drop-back selector 30, the trunk 25, the connector 24 and the subscriber line I2. Y

Subsequently, when the subscriber at the calling subscriber substation TI initiates the release of the established connection by replacing the receiver of the telephone linstrumentupon its associated switchhook, the previously traced loop circuit extending to the connector 24 is interrupted, thereby to cause the connector 24 to be released, whereupon ground potential is removed from the control conductor C28 of the trunk'25. When the connector 24 is thus released the subscriber line I2 is marked as idle to the other connectors having access thereto, assuming that the subscriber at the called subscriber substation T2 has replaced the receiver oi the telephone lnstrument upon its associated switchhook at this time. When ground potential is removed from the control conductor C28 of the trunk 25 and consequently from the control terminal of the drop-back selector 20, the line switch II is released, whereby the subscriber line I0 extending to the callingsubscriber substation TI is marked as idle to the connectors having access thereto.

Also, when ground potential is removed from the l conductor vC20 of the trunk 25, the previously traced holding circuit for energizing the lower winding oi' the'switching relay R00 isinterrupted, thereby vto cause the latter relay to restore. Upon restoring, the switching relay R completes, at the contacts 51, the previously mentioned alternative circuit, including the contacts 12 and 55 and they set of switch springs S52, for energizing the release magnet M45, whereby the latter magnet operates in order to release the wiper shaft 4I and to cause it to be returned to its normal rotary and vertical positions. When the wiper shaft 4I is thus released the sets oi.'

switch springs` S50, S51 and S52 are actuated, as previously explained. More particularly, the set of switch springs S52 is actuated into disengagement, thereby to interruptthe above-traced alternative circuit for energizing the release magnet M49, whereupon the latter magnet restores. At this time the drop-back selector 30 is completely released and available for further use.

In the foregoing explanation of the mode of operation of the drop-back selector 30 in response to the ilrst digit 6 of the directory number dialed at the calling subscriber substation TI, it'was assumed that there was an idle trunk in the first group extendingto the rst group of connectors, including the connector 24. However, it may occur that all of the trunks in the first group are busy at this time when operation of the rotary magnet M41 is initiated. In this event, the rotary n magnet M41 operates intermittently, whereby the wiper shaft 4I is driven eleven steps in the rotary direction away from rits normal rotary position,

wherebyi the sets of switch springs S53 and B54 are actuated. More particularly, the set of switch springs S53 is actuated into engagement, thereby to complete an alternative circuit, including the 4 contactsl 81 and 1 I, for energizing the lower winding of the transfer relay R80 in order to cause the'latter relay to operate. Upon operating, the transfer relay R90 interrupts, at the contacts 0I,

afurther point in the previously traced circuit for energizing the rotarymagnet M41, thereby positively to prevent further operation of the latter magnet at this time. When the set of switch springs S54 is actuated there is interrupted,a't the' contacts 56 thereof, the previously traced loop circuit extending to the calling subscriber substation TI and including the secondary winding of thedial tone transformer l,

and there is completed, at the contacts 55 thereof,

may not be had immediately in view of the al1- busy-condition. In this case, the subscriber at the calling subscriber substation Ti initiates the release of the drop-back selectory 3l kby replacing the receiver of the telephone instrument thereat upon its associated switchhook, whereby the previously mentioned alternative loop circuit for energizing in series the upper and lower windings oi.' the `line relay R10 is interrupted. The line the, contacts 1I, the previously mentioned circuit for energizing the winding of the hold relay R00, whereupon the latter relay restores shortly there' after. Upon restoringthe hold relay R80V inter- 5 rupts. at the contacts 0I, the previously mentioned path forapplying ground potential to the control terminal of the drop-back selector 30 in order to effect the release of the line switch II, in the manner previously explained. Also, upon l0 restoring, the hold relay R00 completes, at the contacts 55, the` previously traced alternative circuit for lenergizing the release magnet M40, whereby the latter magnet operates in order to effect the release of the wiper shaft 4I of the Strowger mechanism 40, in the manner previously noted. The remainder of the release of the drop-back selector 30 incident to the restoration of the hold relay R50 and the operation of the release magnet M49 is the same as that previously described. At this time the drop-back selector 30 is completely released and available for fur' ther use.

In view of the foregoing explanation of the mode of operation of the drop-back selector 00 to select an idle trunk in the first group in response to the first digit 6 of the directory number dialed at the calling subscriber substation Tl, it will be understood that it is operative in a substantially identical manner to select an idle trunk in the second, third or fourth groups in response to the first digits 7, 8 and 9 of the directory numbers dialed at the calling subscriber substation TI. Finally it will be understood that the drop- ,back selector 30 is operative in a manner substantially identical to that previously explained in response to the dialing of the single digit 0 at the calling subscriber substation T I, whereby an idle trunk in the group extending to the groupv of trunk circuits is selected. For example, the drop-back selector 30 may operate in response to the reception of the digit 0 dialed at the calling subscriber substation TI to select the trunk 20 extending to the trunk 'circuit I5, whereby the switching relay R operates in order to com 45 plete an obvious loop circuit between thecalling subscriber substation TI and the trunk circuit I5. 'Ihe trunk circuit I5 then operates in order to signal the operator atthe operator position I3 over the trunk I4 terminating thereat. The oper- 5o ator at the operator position Il then answers the call and communicates with the subscriber at thecalling subscriber substation TI in accordance with conventional practice. The subsequent release of this established connection between the calling subscriber substation TI and the called operator position Il is under the control of the subscriber at the calling subscriber substation TI and is performed in the manner previously explained when the receiver of the telephone instrument thereat is replaced upon its associated switchhook. l

While one embodiment of the invention has been disclosed, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention.

What is claimed is: i 1. In a telephone system, a drop-back selector of the primary-secondary motion type having an incoming trunk and comprising a single unitary contact bank including ilrst and second sections,

said first section including a plurality of spacedapart rows of control contacts and said second section including a plurality of spaced-apart rows of line contacts, av plurality of lines terminated relay R10 then restores, thereby to interrupt, at 76 by said contact bank, each of said lines including control and line conductors respectively -termlnated by corresponding control and line contacts in corresponding rows in said first andtrunk for imparting selective primary movement to said wiper set, whereby said control and line wipers are both moved adjacent corresponding rows of contacts -respectively included in said first and second sections only in the event at least a predetermined amount of primary movement is imparted to said wiper set, means for controlling the return of said wiper set back to its normal position following the primary movement imparted thereto, and means controllable only in the event at least said predetermined amount -of primary movement is imparted to said wiper'set for im-I parting selective secondary movement thereto, whereby said control and line wipers are moved to select corresponding control and line contacts in the adjacent rows respectively included in said iirst and second sections in order to cause said wiper set to select the associated terminated line.

2. In a telephone system, a drop-back selector of the primary-secondary motion type having an incoming trunk and comprising a single unitarycontact bank including iirst and second sections, said first section including a plurality ofspaced-apart rows of control contacts and said second section including a plurality of spacedapart rows of line contacts, a plurality of lines terminated by said contact bank, each of said lines including control and line conductors respectively terminated by corresponding control and line contacts in corresponding rows in said first and second sections, a'wiper set having a normal position and mounted for both primary and secondary movements with `respect to said contact bank, said wiper set including a control wiper and a spaced-apart line wiper adapted to cooperate respectively with said iirst and second sections, means for connecting a dial tone voltage to said incoming trunk, impulse responsive means controllable over said incoming trunk for imparting selective primary movement to said wiper set, whereby said control andline wipers are both moved adjacent corresponding rows of contacts respectively included in said rst and second sections only in the event at least a predetermined amount of primary movement is imparted to said wiper set, means controlled in the event less than said predetermined amount of primary movement is imparted to said wiper set for returning said wiper set back to its normal position, means controllable only in the event at least said predetermined amount of primary movement is imparted to said wiper set for imparting selective secondary movement thereto, whereby said control and line wipers are moved to select corresponding control and line contacts in the adjacent rows respectively included in said first and second sections in order to cause said wiper set to select the associated terminated line, and

means controlled by the `selection of said lastmentioned lineby said wiper set for connecting said incoming trunk to said wiper set and for disconnecting the dial tone `voltage from said incoming trunk. l l

3. In a telephone system, a drop-back selector of the primary-secondary motion type having an incoming trunk and comprising a single unitary contact bank including ilrst and second sections, said rst section including a plurality ofspacedapart rows of \control contacts and said second section including a plurality of spaced-apart rows of line contacts, a plurality of groups of lines terminated bysaid contact bank, each of said groups of lines being terminated vin two corresponding rows in said iirst and second sections and. each of said .lines including control and line conductors respectively terminated by corresponding control and line contacts in the two corresponding rows in said first and second sections, a wiper-set having a normal position and mounted for both primary and secondary .movements with respect to said contact bank, said wiper set including a control wiper and a' spacedapart line wiper adapted to cooperate respectively with said iirst and second sections, means for connecting a dial tone voltage to said incoming trunk, impulse` responsive means controllable over said incoming trunk for imparting selective primary movement to said wiper set, whereby said control and line wipers are both moved adjacent corresponding rows of contacts respectively included in said first and second sections only in the event at least a predetermined amount of primary movement is imparted to said wiper set in order to cause said wiper set to select the associated group of terminated lines, means controlled in the event -less than said predetermined amount of primary movement is imparted to said wiper set for-returning said wiper set back to its normal position, means controllable only in the event at least said predetermined amount of primary movement is imparted to said wiper set for imparting selective secondary movement thereto, whereby said control and line wipers are moved toss-elect corresponding control and line contacts in the adjacent rows respectively inwiper set for connecting said incoming trunk to said wiper set and for disconnecting the dial tone voltage from said incoming trunk, and means controlled in the event a predetermined amount of secondary movement is imparted to said wiper set when there is no idle line in the group previously selected thereby for disconnecting the dial tone voltage from said incoming trunk and for connecting a busy tone voltage thereto.

4. A drop-back selector of the primary-secondary motion type comprising a single unitary contact bank including first and -second sections, said iirst section including a plurality of spacedapart rows of control contacts and said second section including a plurality of spaced-apart rows of line contacts, a wiper set having a normal positlon and mounted for both primary and secondary movements with respect to said contact bank, said wiper set including a control Wiper yand a spaced-apartline wiper adapted to cooperate respectively with said first and :second sections, means for imparting selective primary movement to said wiper set, whereby saidcontrol and line wipers are both moved adjacent corresponding rows of contacts respectively included in said nrst and second sections only in the event at trolling the return of said wiper set back to its normal position following the4 primary movement imparted thereto, and means ior imparting selective secondary movement to said wiper set. whereby said control and line wipers are moved to select corresponding /control and line contacts in the adjacent rowsrespectively included in said rst and second sections.

5. A drop-back selector of the primary-secondary motion type comprising a single unitary contact bank including first and second sections, said iirst section including a plurality of spacedapart` rows of control contacts and said second section including a plurality of spaced-apart rows of line contacts, a wiper set having a normal position and mounted, for both primary and secondary movements with respect to said contact bank, said wiper set including a control Wiper and a spaced-apart line wiper adapted to cooperate respectively with said iirst and second sections, means for imparting selective primary movement to saidwiper set, whereby said control and line wipers are both moved adjacent corresponding rows'of contacts respectively included in said first and second sections only `in the event at least a` predetermined amount of primary movement is imparted to said wiper set, said predetermined amount of primary movement comprising movement of the wiper set away from normal a distance in excess of the distance between said wipers, means for controlling the return of said wiper set back to its normal position following the primary movement imparted thereto, and means controllable only in the event at least said pre determined amount of primary movement is imparted to said wiper set for imparting selective secondary movement thereto, whereby said control and line wipers are moved to select ,corresponding control and line contacts in the adjacent rows respectively included in said iirst and second sections.- i

6. A drop-back selector of the primary-sec- -ondary motion type comprising a single unitary contact bank including first and second sections, said first section including a plurality of spacedapart rows of control contacts and said second section including a plurality of spaced-apart rows of line contacts, a wiper set having a normal position and mounted for both primary and secondary movements with respect to said contact bank, said wiper set including a control wiper and a spaced-apart line wiper adapted to cooperate respectively with said first and second sections, means for imparting selective primary movement to said wiper set, whereby said control and line wipers are both moved adjacent corresponding rows of contacts respectively included in 'said lrst and second sections only in the event at least a predetermined amount of primary movement is imparted to said wiper set, said predetermined amount of primary movement comprising movement of the wiper set away from normal a distance in excess of the distance between said wipers, means controlled in the event less than said predetermined amount of primary movement is imparted to said wiper set for returning said wiper set back to its normal position, and means controllable only in the event at least said predetermined amount of primary movement is imparted to said wiper set for imparting selective secondary movement thereto, whereby said control and line wipers are moved to select corresponding control and line contacts in the adjacent rows respectively included in said iirst and second sections,

7. A drop-back selector of the primary-lecondary motion type comprising a single unitary .contact bank including first and second sections,

said first section including a plurality of spacedapart rows of control contacts and said second section including a plurality of spaced-apart rows of line contacts, a wiper set having a normal position and mounted for both primary and secondary movements with respect to said contact bank, said wiper set including a control wiper and -fa spaced-apart line wiper adapted to cooperate ref spectively with said first and second sections, means for imparting selective step-by-step primary movement to said wiper set, whereby said control and line wipers are both moved adjacent corresponding rows of contacts respectively included in said first and second sections only in the event at least a predetermined plural number of steps of ,primary movement are imparted to said wiper set, means for controlling the return of said wiper setback to its normal position following the primary movement imparted thereto, and means controllable; only in the event at least said predetermined number of steps of primary movement is imparted to said wiper set for imparting selective step-by-step secondary movement thereto, whereby said control and line wipers are moved to select corresponding control and line contacts in the adjacent rows respectively included in said first a'nd second sections.

8. A drop-back selector of the primary-secondary motion type comprising-a single unitary contact bank including iirst and. second sections, said rst section including a plurality of spacedapart rows of control contacts and said second section including a plurality of spaced-apart rows of line contacts, a wiper set having a normal position and mounted for both primary and secondary movements with respect to said contact bank, said wiper set including a contro1wiper and a spaced-apart line wiper adapted to cooperate respectively with said first and second sections, means for imparting selective step-by-step' primary movement to said wiper set, whereby said control and line wipers are both moved adjacent corresponding rows of contacts respectively included in said rst and second sections only in the event at least a predetermined plural number of steps of primary movement are imparted to said wiper set, means controlled in the event less than said predetermined number of steps of primary movement are imparted to said `wiper set for returning said wiper set back to its normal position, and means controllable lonly in the event at least said predetermined number of steps of primary movement is imparted to said wiper set for imparting selective step-by-step secondary movement thereto, whereby said contro1 and line wipers are moved to select corresponding control and line contacts in the adjacent rows respec- Y tively included in said rst and second sections.

9. A drop-back selector of the primary-secondary motion type comprising a single unitary contact bank including rst and second sections.- said ilrst section including a plurality of spacedapart rows of control contacts and said second section including a plurality of spaced-apart rows of line contacts, a wiper set having a normal position and mounted for both primary and secondary movements with respect to said contact bank, said wiper set including a control wiper and a spaced-apart line wiper adapted to cooperate respectively with said rst and second sections, a iirst magnet operative to impart primary mvement to said wiper'set, means for selectively controlling the operation of said nrst magnet, wheredistance eubstentiauy Kv, where K is equal to by said control and line wipers are both moved adjacent corresponding rows of contacts respectively included in said ilrst and second sections only in the event at least a predetermined amount of primary movement is imparted to said wiper set', said predetermined amount of primary movement comprising movement of -the wiper set away from normal a distance in excess of the distance betweensaid wipers, a second magnet operative to' effect the return of said wiper set back to its normal position, a third magnet operative to impart secondary movement to said wiper set, and means controlled in the event less than said predetermined amount of primary movement is imparted to said wiper set for operating said second magnet and controlled in the event at least said predetermined amount of primary movement is imparted to said wiper set for selectiveLv operating said/third magnet, whereby said control and line wipers are moved to select corresponding control and line contacts in the adjacent rows respectively included in said lrst and second sections in the event said third magnet is operated.

10. A drop-back selector of the primary-secondary motion type comprising a singlel unitary contact bank including rst and second sections, said rst section including a plurality of spacedapart .rows of control contacts and said second section including a plurality of spaced-apart rows of line contacts, a wiper set having a normal position and mounted for both primary and secondary movements with respect to said contact bank, said wiper set including a control wiper anda spaced-apart line wiper adapted to cooperate respectively with said iirst and second sections, a iirst magnet operative to impart primary movement to said wiper set, means for selectively controlling the operation of, said first magnet, whereby said control and line wipers are both moved adjacent corresponding rows of contacts respectively included in said rst and second sections only in the event at least a predetermined amount of primary movement is imparted to said wiper set, a second magnet operative to ell'ect the return of said Wiper set back to its normal position, a third magnet operative to -impart secondary movement to said wiper set, a set of springs normally enabling said second magnet and operative t`o disable said second magnet andto enable said third magnet,A means controlled in the event at least said predetermined amount of primary movement is imparted to said wiper set for operating said set of springs, and means controlled shortly following the conclusion of the primary movement of said wiper set for operating said second magnet in the event it is lenabled or for selectively operating said third magnet in the event it is enabled, whereby said control and line wipers are moved 'to select corresponding control and line contacts in the adjacent rows respectively included in said first and second sections in the event said magnet is operated.

11. An automatic switch comprising a single unitary contact bank including a plurality of longitudinally spaced-apart laterally extending rows o1' contacts, wherein each two adjacent rows of contacts are spaced apart a longitudinal distance substantially V and each two -adjacent individual contacts in each row are spaced apart a lateral distance substantially R, a. wiper set associated with said contact bank and mounted for both longitudinal and lateral movements with respect thereto, said wiper set including a control wiper and a line wiper longitudinally spaced apart a third a whole number, means for moving said wiper set longitudinally step'by step a distance :V each step, means for moving said wiper set laterally step by step a distance R each step, whereby the control and line Wipers of said wiper set respectiveiy engage 1n said contact bank two corresponding contacts disposed in two rows longitudinally spaced apart tne distance substantially KV when said wiper set is moved both longitudinally a variable number of steps and laterally a variable number of steps, and a plurality of hnes terminated by said contact'bank, each of said lines including a control conductor and a line conductor respectively terminated in said contact bank by two corresponding contacts disposed in two rows longitudinally spaced apart the distance substantially KV.

12. An automatic switch comprising a single unitary contact bank including a plurality of vertically spaced-apart circumferentially extending rows oi contacts, wherein each two adjacent rows of contacts are spaced apart a vertical distance substantially V and each two adjacent individual contacts in each row are spaced apart a circumferential distance substantially 'said wiper set in the rotary direction step by step a vdistance R each step, whereby thev control and line wipers of said wiper set respectively engage in said contact bank two corresponding contacts disposed in two rows vertically spaced apart the distance substantially KV when said wiper set is moved both in the vertical direction a variable number of steps and in the rotary direction a variable number of steps, and a plurality of lines terminated by said contact bank, each of said' lines including a control conductor and a line conductor respectively terminated in said contact bank by two corresponding contacts disposedin two rows vertically spaced apart the distance substantially KV.

13. In an automatic switch, stationary contacts arranged in a plurality of horizontal rows so that all adjacent rows are spaced apart by the same distance and so that the corresponding end contacts of all rows are in vertical alignment, a pair of wipers disposed one'above the other and separated by a fixed distance equal to an integral multiple of the distance between adjacent rows of said contacts, said wipers normally occupying a position such that both are below the level of the lowest row of said contacts, ele'cr P and adjacent to the'line in which said end con tacts lie, said mechanism eii'ective at times to terminate such movement before the lower one of said wipers has'reached the level of the lowest row of said contacts and effective at other times to terminate such movement when each of said wipers is at the same level as a row of contacts, electromagnetically operated mechanism for moving said wipers horizontally at said other times thereby to bring each wiper into engagement with the end contact of the row with which that wiper is level, and means for disabling said last mechanism at said rst times thereby to prevent such horizontal movement.

14. Inv an automatic switch. stationary contacts arranged ina plurality of parallel rows so that all adjacent rows 'are spaced apart by the same distance and so that the corresponding end contacts of all rows are in alignment. a pair of wipers movable with respect to said stationary contacts but xed in their relationship to each lother, said relationship being such that said wipers are spaced apart from one another by a distance which is an integral multiple of the distance between adjacent rows of stationary con- Vother times.

15. In an automatic switch, two spaced apart wipers, means for moving said wipers as a unit with one wiper trailing the other, two groups oi stationary contacts beside the path travelled by said wipers during such movement,l said groups so related in position to one another and to said path that the leading one of said wipers must pass beside one of said groups during said movement before it reaches a position beside the other group and so that the trailing one of said wipers reaches a position beside said one group at the same time said leading wiper reaches a position beside said other group, means for at times moving said wipers as a/unit in a directacts, means for moving said wipers, with one i 16. In an automatic switch, two spaced apart wipers, means for moving said wipers as a unit with one wiper trailing the other, two groups of stationary'contacts beside the path travelled by said wipers during such movement. said groups so related in position to one another and to said path thatthe leading one oi said wipers must pass beside one oi said groups during said move ment before it reaches a position beside the other group and so that the trailing one of said wipers reaches a position beside said one group at the same time said leading wiper reaches a position beside said other group, said means effective at times to halt said movement when said leading wiper reaches a position beside said one group and eiective at other times to halt said movement when said trailing wiper reaches a position beside said one group, means effective ii said said one group. 

